Field of the Invention
The present invention relates to a modulation/demodulation circuit usable in telephone scrambler or the like and a communication system utilizing such a modulation/demodulation circuit.
Description of the Related Art
There is known a modulation/demodulation circuit which uses a double balanced mixer (DBM). Such a modulation/demodulation circuit is used, for example, in a telephone scrambler.
FIG. 6 shows a modulation/demodulation circuit constructed in accordance with the prior art. The modulation/demodulation circuit generally shown by 10 comprises a DBM 14 which receives an input signal having its frequency (fin) and converts this input signal by the use of a carrier having its frequency (fcar) outputted from an oscillator 12. In other words, the DBM 14 converts the frequency (fin) into .vertline.fcar.+-.fin.vertline.. The output stage of the DBM 14 is connected to an LPF 16 which is a filter for selectively passing only the lower side band (LSB) of the output of the DBM 14.
In the prior art modulation/demodulation circuit, a switch 18 is connected between the DBM 14 and the LPF 16. The switch 18 functions to switch the input of the LPF 16 from the output of the DBM 14 to the input signal or vice versa. For a scrambler, the switch 18 is manually switched by an operator. If the switch 18 is moved upwardly as viewed in FIG. 6, the circuit is placed in a modulation/demodulation mode wherein the output of the DBM 14 is fed to the LPF 16. If the switch 18 is moved downwardly, the circuit is placed in a through mode wherein the input signal is applied directly to the LPF 16 not via the DBM 14. Only in the modulation/demodulation mode, may the input signal to the LPF 16 be scrambled or unscrambled by the actuation of the DBM 14.
FIG. 7 shows a system connecting modulation/ demodulation circuits 10-T and 10-R by a transmission channel 20. Each of these modulation/demodulation circuits 10-T and 10-R has the same arrangement as that of the modulation/demodulation circuit 10 shown in FIG. 6.
FIGS. 8a-8e show signal conditions at various points in the circuit wherein modulation/demodulation mode, that is, a mode when switches 18-T and 18-R are shifted upwardly as viewed in FIG. 8. When an input signal is applied to the transmission-side modulation/demodulation circuit 10-T as shown in FIG. 8a, the output of a transmission-side DBM 14-T is frequency converted, as shown in FIG. 8b. More particularly, the frequency (fin) is converted into a frequency.vertline.fcar.+-.fin.vertline.. A symbol ".fwdarw." in FIG. 8 represents that the frequency components are arranged in the order of the original signal frequencies while a symbol ".rarw." shows that the frequency components are arranged in the opposite order.
In the modulation/demodulation mode, a signal having frequency characteristic such as are shown in FIG. 8b is inputted to the LPF 16-T since the switch 18-T is shifted upwardly. The LPF 16-T has a cut-off characteristic which causes only frequency components having frequencies less than the carrier frequency (fcar) to be passed to the transmission channel 20. Namely, the LPF 16-T has a characteristic such as is shown by a broken line in FIG. 8c. The signal fed from the LPF 16-T to the transmission channel 20 will contain only the LSB of the output of the LPF 16-T, as shown in FIG. 8c. The LSB signal is a signal whose frequency components are arranged in the inverse order to the original signal.
As the output of the LPF 16-T is provided to the reception-side modulation/demodulation circuit 10-R through the transmission channel 20, DBM 14-R receives this signal and frequency converts it as in the transmission-side DBM 14-T. Thus, the output of the DBM 14-R becomes such a signal as is shown in FIG. 8d.
The output of the DBM 14-R is inputted to LPF 16-R through the switch 18-R. The LPF 16-R has the same cut-off characteristic as that of the LPF 16-T. Therefore, only the LSB of the output of the DBM 14-R will be outputted from the LPF 16-R. Namely, the output of the reception-side modulation/demodulation circuit 10-R will be such a signal as is shown in FIG. 8e.
In the through mode, tile switches 18-T and 18-R are shifted downwardly as viewed ill FIG. 7. The DBMs 14-T and 14-R are inoperative in the through mode. As a result, such a signal as is shown in FIG. 9a is directly input to the transmission-side LPF 16-T which in turn converts it into a signal having such a frequency as is shown in FIG. 9b. As this signal is applied to the reception-side LPF 16-R through the transmission channel 20, the reception-side LPF 16-R will output a signal having such a frequency characteristic as is shown in FIG. 9c.
In such a manner, an operator can manually shift the prior art modulation/demodulation circuit from the modulation/demodulation mode to the through mode or vice versa.
In such an arrangement, however, the frequency characteristic of the output signal will vary between the modulation/demodulation mode and the through mode. In the scrambler, the tone quality in the same output signal will become different between the modulation/demodulation mode and the through mode, for example.
More particularly, the output signal shown in FIG. 8e becomes a signal having a substantially trapezoid-shaped frequency characteristic with the components near DC being cut off, since in the modulation/demodulation mode the output of the transmission-side LPF is frequency inverted by the reception-side, as shown in FIGS. 8c and 8d. On the contrary, the frequency will not be inverted by the reception-side DBM in the through mode. Thus, the cut-off shape inverted by the transmission-side DBM will not appear at the lower band range. In other words, the frequency characteristic is flat towards the lower band range, as is shown in FIG. 9c.